1. Field of the Invention
The present invention relates to phase tracking, and more particularly, to phase tracking for carrier recovery in a demodulator of a telecommunications receiver.
2. Description of the Related Art
Many telecommunications applications require carrier recovery and demodulation by a receiver. One telecommunications application is reception and demodulation of High Definition Television (HDTV) signals, as proposed in the Advanced Television Standards Committee (ATSC) advanced television (ATV) standard. HDTV signals may include vestigial sideband (VSB) component signals representing code symbol levels, and for these HDTV signals the pilot carrier is a waveform having discrete, fixed amplitudes. Each amplitude corresponds to a prescribed percentage of modulation of the carrier, and is desirably the same percentage as that associated with the smallest change in code symbol level. Code symbol levels may typically be 8 or 16. Although such VSB signals may be used, for example, in over-the-air broadcasting of HDTV signals, VSB signals may be used in many other telecommunications applications, such as in cable-casting systems.
Techniques to demodulate the HDTV signals have been proposed, such as those described in ATSC ATV standard document Doc. A/54, xe2x80x9cGuide to the Use of the ATSC Digital Television Standard.xe2x80x9d Proposed radio receivers may employ double-conversion followed by synchronous detection. According to this technique, a frequency synthesizer generates first local oscillations that are heterodyned with the received television signals to generate first intermediate frequencies (e.g., with a 920 MHz carrier). A passive, bandpass filter selects these first intermediate frequencies for amplification by a first intermediate-frequency amplifier. The amplified first intermediate frequencies are then filtered by a first surface-acoustic-wave (SAW) filter that rejects adjacent channel responses. The first intermediate frequencies are heterodyned with second local oscillations to generate second intermediate frequencies (e.g., with a 41-MHz carrier). A second SAW filter selects these second intermediate frequencies for amplification by a second intermediate-frequency amplifier. The response of the second intermediate-frequency amplifier is synchrodyned to baseband with third local oscillations of fixed frequency.
The third local oscillations of fixed frequency are supplied in 0xc2x0- and 90xc2x0-phasing for the in-phase and quadrature-phase synchronous detection process (I-phase and Q-phase detection). I-phase detection provides the eight-level code symbols of the broadcast HDTV VSB signals as a result. Q-phase detection provides a nominally zero-valued result. However, when digital sampling of the signals is employed, the separate processes of I-phase and Q-phase detection may cause several problems. For example, the I-phase and Q-phase detection results may not necessarily track each other after sampling, and quantization noise may introduce phase errors in the corrected signal (when considered as a phasor).
Separate I-phase and Q-phase detection may be implemented by digitally sampling the output signal of the second intermediate-frequency amplifier at twice the Nyquist rate of the eight-level coding. The successive samples are considered to be consecutively numbered in order of their occurrence. Odd samples and even samples are separated from each other to generate respective ones of the I-phase and Q-phase detection results. The result of the I-phase detection containing the eight-level coding may be filtered to remove co-channel interference from NTSC signals. The result of the I-phase detection is subjected to equalization from an equalization filter before being applied to a trellis decoder. The trellis decoder response is interleaved data provided to a de-interleaver. The de-interleaver supplies the data to a Reed-Solomon decoder.
Synchrodyning may be employed to recover the modulating signal at baseband (baseband extending from zero frequency to the highest frequency in the modulating signal). Synchrodyning for I-phase and Q-phase detection employs the result of Q-phase detection to generate automatic-frequency-and-phase-control (AFPC) signals. A controlled oscillator employs the AFPC signals to adjust the frequency and phase of the second local oscillations. Adjusting the frequency and phase of the second local oscillations reduces the amplitude, and hence, error, of the Q-phase detection result.
This automatic frequency and phase control of the prior art may be inadequate in providing the desired degree of phase stability for I-phase detection. An equalization filter may be used to correct for static phase error of the synchrodyning process by adaptively filtering the result of I-phase detection. However, changing filter coefficients of the equalization filter may occur too slowly to compensate for phase jitter in the AFPC feedback loop or to compensate for the changes in phase error that occur during rapid changes in multipath reception of the HDTV signal.
Accordingly, in HDTV signal radio receivers of the prior art, a phase tracker is cascaded with the equalization filter. The phase tracker of the ATSC ATV standard performs a Hilbert-transform with a finite-impulse-response (FIR) filter (a Hilbert-transform filter, or HTF) and then uses a phase-locked loop (PLL) for phase de-rotation. The equalized result of the I-phase detection is supplied as a digital signal to the Hilbert-transform filter. The response of this Hilbert-transform filter and the equalized result, as delayed to compensate for the latency of the Hilbert-transform FIR filter, are applied as real and imaginary input signals to a complex-number multiplier to generate a complex-number product. These real and imaginary input signals may be defined with coefficients and a unit Euler vector.
A feedback loop ascertains the departure from the zero axis of the imaginary component of the complex-number product to develop an error signal for adjusting the phase angle of the unit Euler vector. The real and imaginary values of the unit Euler vector are drawn from a sine/cosine look-up table (LUT) stored in read-only memory (ROM). The sine/cosine LUT is addressed by the output of an accumulator used for integrating the error signal. However, this phase tracker requires LUT entries to generate an estimate of the phase error, which adds both computational steps and complex circuitry if reasonable accuracy is desired.
In a modified HDTV signal radio receiver, the second local oscillations are heterodyned with the first intermediate frequencies to convert them to second intermediate frequencies, and the second local oscillations are of a fixed frequency. Accordingly, phase jitter of the AFPC feedback loop of a controlled oscillator is eliminated. The second local oscillations are at a fixed frequency offset from the frequency of the carrier for the second intermediate frequencies, and the second local oscillations are heterodyned with the first intermediate frequencies to downconvert them to third intermediate frequencies. The third intermediate frequencies still exhibit changes in multipath reception of the HDTV signal requiring a phase tracker. The phase tracker is implemented before adaptive filtering with an equalization filter, and may be a bandpass phase tracker, rather than a baseband (or lowpass) phase tracker, as described previously. The bandpass phase tracker, however, exhibits similar problems of the baseband phase tracker.
The present invention relates to phase tracking of a signal component to form a corrected signal by providing a quadrature component from the signal component and applying a linear transform, based on a set of coefficients, to the signal component and the quadrature component. The set of coefficients is generated based on a previous set of coefficients and a previous corrected signal, the set of coefficients being derived in accordance with an optimization criterion. For a preferred embodiment, the optimization criterion is based on the minimum mean square error between the ideal channel signal and the corrected signal. One advantage of the present invention is that the phase tracking not only performs a phase rotation of the constellation, but also performs a gain adjustment. Also, the phase tracking is optimal with respect to an error criterion, such as mean squared error, and the phase tracking does not require use of a sine/cosine lookup table.